1. Field of the Invention
The present invention relates to a system for detecting a load applied to a suspension. More specifically, the invention relates to a load detecting system, in which a strain gauge is provided.
2. Description of the Prior Art
A system for detecting a load, which is applied to the suspension of an automotive vehicle, a motor cycle and so forth, is described in the Japanese Patent First Publication (Jikkai) No. Showa 57-37008. This system includes a pair of strain gauges, which are mounted on shafts connecting the ends of suspension assemblies of a two-wheeler with the vehicle frame, and a pair of fixed resistors. The strain gauge and the fixed resistance constitute a Wheatstone bridge which detects load applied to an air suspension. In this system, not only the usual components of the suspension but also a supporting shaft, to which the strain gauge can be secured, are required in order to detect a load. Therefore, the number of parts is large and its construction is complicated. In addition, a strain gauge is attached to both ends of the suspension assembly, i.e. to the supporting shafts of both of the sprung and the unsprung mass, respectively. One of the strain gauges is separated from the other by a great distance. Therefore, it is difficult to compensate for temperature difference between the strain gauges. Furthermore, the durability of each strain gauge is not very good since it is attached by means of an adhesive agent. In this system, the strain gauges are attached to the suspension assembly after the suspension assembly is mounted on the vehicle body. Therefore, the installation procedure is complicated and it is difficult to determine the optimum mounting point. In addition, it is difficult to establish a lead wire from the strain gauge of the movable unsprung mass to the sprung mass, that is not influenced by the displacement of the suspension.
Another system for detecting a load applied to the suspension of an automotive vehicle is described in the Japanese Patent First Publication (Jikkai) No. Showa 60-106807. This system includes an elastic body provided between a suspension spring and the vehicle body. The elastic body is comprised of, for example, an insulating rubber, on a part of which a conducting rubber layer is formed. This system may detect a load applied to the suspension by the variation of the resistance value of the conducting rubber layer according to the deformation of the elastic body. However, a certain amount of carbon black is added to the rubber used for elastic members for automotive vehicles in order to give it the required hardness and durability. Therefore, this causes the rubber to have some conductivity. The resistance value or the conductivity of the rubber varies according to the ratio and compounding manner of the carbon black and rubber. In general, there is about 20% to 30% variation in conductivity between samples. In addition, the conducting rubber layer may alternatively include a metal strip, the conductivity of which varies under the same conditions as described above. Therefore, when such a member is used as a detecting member, it is difficult to accurately detect a load applied to the suspension on the basis of variation of the conductivity, i.e. variation of the resistance since the conductivity of the elastic member varies between samples and the proportion of the variation is not constant.
Another system is further described in the Japanese Patent First Publication No. Showa 59-43205. This system includes a load detecting device comprised of a piezo-electric device which is mounted on the upper end of a shock absorber. The piezo-electric device is fixed to the vehicle body with the shock absorber when the shock absorber is mounted on the vehicle body. The system may detect load applied to the suspension by means of electric signals produced from the piezo-electric device. However, loads on the basis of the upward and downward movement of the suspension while driving the vehicle are continually applied to the position where the piezo-electric device is disposed. Therefore, looseness due to wear may occur at the thread fastening point decreasing the sensitivity of load detection after a number of years. In addition, although the system may detect the varied value of the applied loads, it can not detect continuous loads or the absolute value of the loads.